all the HTTT ones are on Com:lag (i think, i do have all those in single form too, but it would be easier to get the eps)

as for the older ones, you can get some from specialist places, but even the reissuses are about £7 a pop so it would be pretty expensive, so i'd ask on the ateasse forum for someone to link you up all their b-sides

* Itch - some Pablo Honey-era b-sides, easier to find these days than it used to be but probably the 'rarest'.
* My Iron Lung - My Irong Lung + the six b-sides from its UK CD singles + something else I forget, been difficult to find to the extent that a lot of people seem to think it's a domestic release.
* Airbag/How Am I Driving? - bunch of OK Computer era b-sides, used to be quite hard to find by has become a lot easier since they left EMI, hello stock dumping!
* Com Lag - all the HTTT era ones, Japanese only originally but again very easy to find here since they left EMI.

Not complete by any shot though. But I would expect a proper b-sides release to appear at some point, it's clearly going to be a money winner for EMI.